Fix PlotItem.setAxisItems (#1376)

* Fix PlotItem.setAxisItems

- Use extend so visibleAxes remains a flat list.
- More robust logic for detecting adding an AxisItem instance to
  mulitple plots and suggest a workaround in the error message.

* Simplify membership check

* Add test for PlotItem setAxisitem logic

pyqtgraph/graphicsItems/PlotItem/PlotItem.py

@@ -307,7 +307,7 @@ class PlotItem(GraphicsWidget):
         # Array containing visible axis items
         # Also containing potentially hidden axes, but they are not touched so it does not matter
         visibleAxes = ['left', 'bottom']
-        visibleAxes.append(axisItems.keys()) # Note that it does not matter that this adds
+        visibleAxes.extend(axisItems.keys()) # Note that it does not matter that this adds
                                              # some values to visibleAxes a second time
         
         for k, pos in (('top', (1,1)), ('bottom', (3,1)), ('left', (2,0)), ('right', (2,2))):
@@ -325,8 +325,11 @@ class PlotItem(GraphicsWidget):
             if k in axisItems:
                 axis = axisItems[k]
                 if axis.scene() is not None:
-                    if axis != self.axes[k]["item"]:
-                        raise RuntimeError("Can't add an axis to multiple plots.")
+                    if k not in self.axes or axis != self.axes[k]["item"]:
+                        raise RuntimeError(
+                            "Can't add an axis to multiple plots. Shared axes"
+                            " can be achieved with multiple AxisItem instances"
+                            " and set[X/Y]Link.")
             else:
                 axis = AxisItem(orientation=k, parent=self)
             

pyqtgraph/graphicsItems/PlotItem/tests/test_PlotItem.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+import pytest
+import pyqtgraph as pg
+
+app = pg.mkQApp()
+
+
+@pytest.mark.parametrize('orientation', ['left', 'right', 'top', 'bottom'])
+def test_PlotItem_shared_axis_items(orientation):
+    """Adding an AxisItem to multiple plots raises RuntimeError"""
+    ax1 = pg.AxisItem(orientation)
+    ax2 = pg.AxisItem(orientation)
+
+    layout = pg.GraphicsLayoutWidget()
+
+    pi1 = layout.addPlot(axisItems={orientation: ax1})
+
+    pi2 = layout.addPlot()
+    # left or bottom replaces, right or top adds new
+    pi2.setAxisItems({orientation: ax2})
+
+    with pytest.raises(RuntimeError):
+        pi2.setAxisItems({orientation: ax1})